The invention relates to a zone melting process wherein a charge is heated by an electrical current conducted through the charge.
In German Pat. No. 1,169,683 there is described a process for the zone melting of a semiconductor rod without the use of a crucible or boat wherein the molten zone is produced by a radiant energy source directed at right angles to the axis of the semiconductor rod. The energy source surrounds the rod concentrically and an approximately level course is produced for the boundaries between the molten zone and the solid semiconductor material supporting it. In addition, heating of the semiconductor rod is performed by a current flowing through the rod in a longitudinal direction. In this manner, it is intended to achieve a general, perferably uniform, heating so that the temperature achieved by this heating is below the melting point of the rod. The object of this known process is the production of a boundary between the liquid and the solid phases as level as possible. However, because of the particular way in which heat is supplied to the melting zone, the expense required is considered. Furthermore, difficulties arise if the surface tension of the molten material is not maintained at an adequate level. Finally, the energy expenditure and investment costs are high and the process, in general, is relatively uneconomical.
An older German Pat. application P 21 25 726.1 discloses a zone melting process wherein an elongated charge of melted material has a temperature above the melting point of the material in a zone migrating in the direction of the longitudinal direction of the charge. After the migrating zone, the material has a temperature at least below the melting point of the material. A particular feature of this known process is that the current flowing longitudinally through the charge is capable of heating the charge to a temperature above the melting point. Further, the charge, at least in the region positioned after the zone, in the direction of travel of the charge, is cooled to a temperature below the melting point. In this way, the introduction of energy for producing and maintaining the melting zone is not necessary, as was the case with the process of the 1,169,683 Pat. Specification. In the older process, the current heats the charge in the region of the melting zone above the melting point and, outside the melting zone, suitable cooling is provided to ensure that the liquid phase merges into the solid phase, or that the charge remains unmelted. This process, since only one heat source is required and the cooling effect can be readily achieved e.g. by radiation and/or by air, affords a great simplicity as regards the arrangement for carrying out the process, and is very economical. However, difficulties may result in melting materials having inadequate surface tension since a certain surface tension is required to hold the material together in the melting zone.
The known Verneuil process permits the growing of crystals from materials of low surface tension, since the liquid layer formed on the drawn crystal rod on which particles are deposited can be kept very thin, so that there is no risk that the liquid will flow away laterally. Preferably, material from which the rod is grown is in the liquid state.
A modification of the Czochralski method is known wherein the tip of a finished rod is melted and the crystal is pulled from the resultant melt. With this method, also known as the "pedestal-growth" method, it is possible for outflow to occur if the liquid in the melting zone is of low surface tension. Therefore, this method is suitable only for materials of adequate surface tension.